The 12 volt systems used in today's automobiles are required to supply ever increasing currents as the load on the system continues to increase. This increase is due to a combination of increasing numbers of electronic devices, such as communication, entertainment, and telematics systems, as well as the proliferation of electrically powered auxiliary systems to replace traditional hydraulically or mechanically powered systems (e.g., electric power steering). To reduce the amount of current required to supply these higher loads, it has been proposed that automobiles should adopt 42 volt electrical systems. The automotive industry, however, has been reluctant to transition to 42 volt electrical systems because of increased costs. Consequently, there is a strong demand to improve the performance of 12 volt systems, thereby allowing higher electrical loads to operate effectively with conventional vehicle electrical systems.
One limiting factor on automotive electrical system performance is the voltage drop that occurs across the rectifier bank when the alternating current (AC) voltage generated by the alternator is converted to a direct current (DC) voltage. Diode rectification is the predominant method of AC/DC conversion. The forward voltage drop of the diodes (typically about 0.7 volts) is a significant proportion of the battery voltage. Consequently, it has a major impact on system efficiency and also reduces the current drawn from the alternator. Diodes having a lower voltage drop or higher system voltages mitigate this problem but neither solution appears likely to reach commercialization in the near future. The efficiency of automotive electrical systems, however, may be increased by using field effect transistors (FETs) to actively rectify alternator output, rather than diodes currently used for passive rectification. The voltage drop across a switched-on FET (as low as 0.1 volt) is substantially lower than the drop across a forward biased diode.
One problem with FET-based rectifiers is that of determining when to turn the switches on and off. Historically, most techniques have used an alternator shaft position sensor to determine when the FETs should switch. Unfortunately, alternator manufacturers generally do not provide such a sensor as part of a standard alternator, and custom parts include attendant higher costs. Other prior art techniques have used an estimation technique that requires three current sensors, one on each AC phase. For cost reasons, the use of multiple AC current sensors is not feasible in automotive electrical system applications.
In other applications where FETs are used to rectify AC electricity, such as single phase and three phase AC power supplies, the source voltage tends to be very stiff, i.e., the source voltage is not affected by the amount of current drawn therefrom. Because inductors are typically included in such systems to reduce current harmonics, the voltages across these inductors are readily available for switching control. Unfortunately, in an automotive alternator, the inherent inductance in series with each winding is significant, resulting in a substantially less stiff voltage source. As a result, the terminal voltage is distorted considerably as current is drawn from the alternator, and control techniques from the AC power supply arts are not easily applicable.
Thus, what is needed in the art is a method for properly controlling a FET-based rectifier in an automotive application without the use of a position sensor or the use of AC current sensors on each alternator phase.